The alignment of an inertial navigation system is essential for accurate performance. Normally, several different alignment modes are available to the system operator depending on the amount of time that is available before navigation must begin. For a system carried by an aircraft a typical set of gimballed inertial system alignment modes may include a single position ground alignment, a two position (gyrocompass) ground alignment, and an extended ground alignment involving multiple platform orientations with respect to local gravity and the earth rate vector. A two position alignment can provide adequate performance as it allows calibration of both level gyro biases.
However, for strapdown inertial navigation systems alignment accuracy is more difficult to achieve. The heading can be determined only to the accuracy of the effective east/west component of level gyro bias or, conversely, the effective east/west gyro bias can ascertained only to the accuracy allowed by the input heading. In addition, inertial attitude errors have a much more severe impact on strapdown inertial navigation system performance than for gimballed systems as total vehicle body rates drive cross axis tilt errors and misalignment sensitivities. It is therefore of great benefit for strapdown inertial navigation systems to employ additional measurements to compensate for their inherent lack of calibration flexibility.
As can be seen in FIG. 6 star tracker 1 is a particularly useful calibration aid for augmenting a strapdown inertial system 2 in that the star tracker 1 accurately observes system alignment errors, including strapdown inertial navigation system maneuver-induced errors. A star tracker and filter combination provides gyro drift and scale factor corrections, accelerometer bias corrections, tilt corrections, velocity corrections and position corrections to the strapdown Inertial Measurement Unit (IMU) 2. Furthermore, the star tracker 1 is self-contained, need not increase an aircraft's signature, and is not susceptible to hostile jamming.
Some conventional star trackers have a telescope that images one region of the sky at any given time. In order to view a plurality of stellar objects it is necessary to reposition the telescope. One technique repositions the telescope or the telescope's field of view (FOV) relative to the frame of the vehicle. However, this technique requires precision pointing apparatus, such as a gimballed platform, that adds to the cost and complexity of the star tracker and furthermore may in itself introduce a positional error. Such a positionable telescope is not considered to be a strapdown star tracker system.
The telescope may also be repositioned by movement of the vehicle itself. By example, the telescope may be fixed, or strapped down, to the frame of a satellite while the orbital and/or spin rate of the satellite is used to acquire different stars. While suitable for use in some types of satellite and missile applications this latter technique is generally not applicable to aircraft, especially high velocity aircraft, operated within the atmosphere.
Strapdown star trackers are also known that employ a plurality of smaller telescopes each pointing at a different area of the sky. A disadvantage of this type of system is that the effective entrance aperture, for a given size and weight of the star tracker, is divided among the plurality of telescopes. Thus, each of the plurality of telescope entrance apertures is smaller than that of a single telescope of equivalent aperture and, therefore, the light gathering capability and sensitivity of the system is compromised.
It is thus one object of the invention to provide a strapdown star tracker having a telescope that simultaneously views a plurality of regions of the sky.
It is another object of the invention to provide a strapdown star tracker having a telescope that employs a multiple field of view holographic lens as an input lens.
It is another object of the invention to provide a strapdown star tracker having a telescope that employs a single or a multiple field of view positive holographic optical element at an input aperture and that corrects for a chromatic aberration induced by the positive holographic optical element with one or more negative holographic elements.